Maybe understanding droid is something the Jedi teach, along with the other skills. Poe would have learned it growing up with a pilot mom and when he trained it would probably come up. Rey might have learned it from Unkar Plot. Luke might not have had the time to learn it in the OT.
Regardless of who exactly Rey learned it from it is stated in the Visual Dictionary that she learned the language of the Wookies and Droid Binary during her time on Jakku. So in that department Rey is in the clear as far as that Mary Sue stuff goes.
I think the criticism is that she can do everything, and knows everything, so giving an explanation for why she knows this or knows that, isn’t exactly addressing the criticism.
From my recollection a Mary Sue is a character who is good at everything with no explanation and for the most part that doesn’t fit Rey. If you want to leave the Mary Sue shit out of this and talk about competency in general you would certainly have a case that Rey is really competent but she’s not a Mary Sue. She definitely has a couple Mary Suish traits though.